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No exploitation reportedFix available

CVE-2026-27205

No exploitation reported by the sources we read.

  • Published2026-02-21
  • Fixed in3.1.3
  • WeaknessCWE-524

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

Flask is a web server gateway interface (WSGI) web application framework. In versions 3.1.2 and below, when the session object is accessed, Flask should set the Vary: Cookie header., resulting in a Use of Cache Containing Sensitive Information vulnerability. The logic instructs caches not to cache the response, as it may contain information specific to a logged in user. This is handled in most cases, but some forms of access such as the Python in operator were overlooked. The severity and risk depend on the application being hosted behind a caching proxy that doesn't ignore responses with cookies, not setting a Cache-Control header to mark pages as private or non-cacheable, and accessing the session in a way that only touches keys without reading values or mutating the session. The issue has been fixed in version 3.1.3.

Timeline

dates in UTC
  1. 2026-02-2106:17CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 21:37 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2026-27205?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2026-27205?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD · OSV
Flask
BranchAffectedFixed in
3.1< 3.1.33.1.3

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Vendor advisories

  • CVE-2026-27205 Red Hat Product Security8 known affected

    flask: Flask: Information disclosure via improper caching of session data

    rhoai/odh-kf-notebook-controller-rhel8 as a component of Red Hat OpenShift AI (RHOAI) · rhoai/odh-notebook-controller-rhel8 as a component of Red Hat OpenShift AI (RHOAI) · quay/quay-rhel8 as a component of Red Hat Quay 3 · +5 more

Full record on NVD

Fix

  1. Update Flask to 3.1.3
  2. No fix available, says the vendor Red Hat Product Security CVE-2026-27205 · 5 productsFix deferred
  3. No fix planned, says the vendor Red Hat Product Security CVE-2026-27205 · 3 productsOut of support scope

Advisories and patches

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
none listed
PoC-in-GitHub

Sources

  • CISA-ADPSSVC decisionnone2026-02-24
  • OpenCVESSVC decisionnone—
  • CNACVSS 4.02.3read 2026-10-23
  • NVDCVSS 3.14.3read 2026-10-23
  • CNAweakness (CWE)CWE-524read 2026-10-23
  • FIRST EPSSEPSS score0.5 %2026-10-23
  • OSVaffected packages—

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N
CVSS 4.0 vector
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Published
2026-02-21
Last Modified
2026-06-17
NVD status
Analyzed

Frequently asked questions

What is CVE-2026-27205?

Flask is a web server gateway interface (WSGI) web application framework. In versions 3.1.2 and below, when the session object is accessed, Flask should set the Vary: Cookie header., resulting in a Use of Cache Containing Sensitive Information vulnerability. The logic instructs caches not to cache the response, as it may contain information specific to a logged in user. This is handled in most cases, but some forms of access such as the Python in operator were overlooked. The severity and risk depend on the application being hosted behind a caching proxy that doesn't ignore responses with cookies, not setting a Cache-Control header to mark pages as private or non-cacheable, and accessing the session in a way that only touches keys without reading values or mutating the session. The issue has been fixed in version 3.1.3.

Is CVE-2026-27205 actively exploited?

Active exploitation of CVE-2026-27205 has not been confirmed. Its EPSS score was 0.5% on 2026-10-23, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2026-27205?

CVE-2026-27205 has a CVSS v3 base score of 4.3 (MEDIUM severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N.

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